The present invention relates to an apparatus and method to atomize a liquid to form charged particles, which are concentrated into a smaller volume of carrier gas and then deposited onto a substrate. An electric field is used to aid in uniform deposition of the particles.
Technologies for creating thin films on surfaces are basic to the semiconductor industry. Thin films of oxides, nitrides, ceramics, and other materials are used as insulating layers in thin film transistors as part of large-scale integrated circuits (IC) devices. Such devices are used as microprocessors, volatile and non-volatile memories, digital signal processing chips, among others. Conductive metal thin films must also be deposited and be used as conductive pathways, or inter-connects, among thin-films transistors used in an IC or for connection to external circuits. Photo-sensitive materials, which are commonly called photoresists, must also be deposited in thin film form during IC fabrication in order to create the needed geometrical patterns by photo-lithography.
Technologies for thin film deposition are well developed for silicon, and other semiconductor materials, such as gallium arsenide. Commonly used methods for thin film deposition include chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), election beam (E-beam) evaporation and sputtering. In all of these methods, the precursor chemicals used in creating the thin films are produced in vapor form. Atoms or molecules of the vapor are then deposited on the semiconductor wafer surface to create the thin film, the exception being photoresist thin films, which are usually produced by the spin-coating process, in which a solution containing the photoresist and a volatile solvent is first applied to a spinning wafer to form a liquid thin film. The volatile solvent is then allowed to evaporate to produce a photoresist thin film on the wafer.
The technologies described above for fabricating thin film transistors on semiconductor wafers have also been adapted for use in making flat-panel displays. The substrates used for flat-panel displays are usually glass and the areas of the substrate are also much larger than that of the wafer. Currently, the largest wafer used in the commercial production of semiconductors is 8 inches (200 mm) in diameter, although wafers of 12 inches (300 mm) diameter are expected to be in wide-spread commercial use beginning in 1999 or in the year 2000. In comparison, flat-panel displays usually involve substrate areas that are much larger. For use as a television (TV) monitor, a flat-panel may need to be as large as 60xe2x80x3 in diagonal measure to compete with a TV made with a conventional picture tube. Such large area flat panel displays must be made with a process that is economical and capable of being used for large scale commercial production.
There are two major technical difficulties in producing large area flat panels by means of the convention thin film deposition process. One difficulty is that the conventional CVD, PECVD, E-beam evaporation and sputtering process must be carried out in vacuum. The vacuum typically ranges from a few milli Torr to a few Torr in pressure. To carry out the process in vacuum, a vacuum chamber must be large enough to contain the substrates while carrying out the deposition process. The substrates must first be introduced into the vacuum chamber for thin film processing, and then be removed from the vacuum chamber for additional processing. Large vacuum chambers are expensive to build and operate, and the need to introduce and remove substrates from a vacuum chamber slows down the process, making it very expensive to make large flat panels. A second difficulty relates to the operation of the CVD, PECVD, E-beam evaporation, and sputtering processes, all of which involve depositing one molecule or atom at a time. The process is too slow except for the very thin films that can be used in semiconductors.
Other thin film deposition process have also been tried in the past for semiconductor fabrication. Drakitchiev U.S. Pat. No. 4,996,080 describes a photoresist coating process in which the photoresist solution is atomized to form a spray. The spray droplets are then introduced into a chamber and allowed to settle by gravity onto a substrate at the bottom of the chamber. The substrate is then spun to form a uniform coating of photoresist on the substrate. Donovan et al. U.S. Pat. No. 5,229,171 describes a similar approach in which a photoresist spray is first created and then allowed to deposit on the substrate in a vacuum. Electrodes are incorporated in the apparatus to create an electric field to aid in the deposition process.
The need to create thin films of complex chemicals with special electrical properties has led some inventors to develop thin film deposition by droplets. One such invention is that described by McMillan et al. in U.S. Pat. No. 5,316,579, in which a fine mist is formed by a rotating turbine blade within an enclosure. The mist is then withdrawn under vacuum and allowed to deposit onto a substrate in vacuum by gravity. Methods of creating thin films of ferroelectric, super-conducting, and high dielectric thin films by the method are claimed. Other patents such as U.S. Pat. Nos. 5,456,945, 5,540,772, 5,614,252, and 5,688,565 relate to the improvement of the so-called Liquid Source Misted Chemical Deposition (LSMCD) process and application of the process for creating thin films of barium strontium titanate (a high dielectric constant material) or layered superlattice materials, among others. In all of these inventions, the substrate is placed in a vacuum chamber, and the deposition is also carried out in vacuum, again leading to slow processing and expensive equipment.
As discussed just above, a variety of methods have been developed to atomize liquid to form droplets. Centrifugal atomizers, ultrasonic nebulizers, pump sprayers, and compressed air atomizers/nebulizers are some examples of atomization or nebulization devices that are used in spray painting, in personal care products, such as hair sprayers, deodorizers, and drug delivery devices to deliver drugs in aerosol form into the human lung. Most of these devices produce droplets that are either uncharged or carry only a weak natural electrical charge. The most well known exception is electrostatic spray painting in which a high electrostatic charge is placed on the atomized paint droplets by applying a high voltage to the paint during atomization. Because the paint is held at a high voltage, the paint must be electrically non-conductive. This restricts the conventional electrostatic spray guns to organic solvent based paints. When water based paints such as latex paints and other paints consisting of dispersion of paint in water is to be sprayed, the conventional electrostatic spray guns cannot be used.
In integrated circuit applications, there is considerable interest in recent years in applying thin films onto a semiconductor wafer surface using aerosol deposition processes. These processes can also be used to provide thin film deposition in manufacturing flat-panel displays. Typically, the precursor chemical used in the aerosol deposition process is a chemical compound dissolved in a solvent or a complex mixture of several chemical species dissolved in a solvent. To preserve the chemical nature of the material or the composition of the desired chemical elements in the thin film, the liquid solution is atomized to form a droplet aerosolxe2x80x94also referred to as a mistxe2x80x94which in turn is deposited onto the surface. Conventional chemical vapor deposition using a vapor source does not work well with such materials because the vaporization process often alters the chemical nature or the mixture composition which can lead to inferior quality films. An example of the misted deposition process used in fabricating ferroelectric and other high dielectric thin films such as BST (barium, strontium titanate) or SBT (strontium bismuth titanate is that described McMillan et al. U.S. Pat. No. 5,456,945, also mentioned above.
In contrast to the above, insulating thin films of a low dielectric constant are also of considerable interest. Such thin films are needed to replace the silicon dioxide and silicon nitride thin films currently in use in integrated circuit devices. Low dielectric constant thin films will increase the speed of microprocessors, memory and other electronic devices. Polymers are considered ideal because of their low dielectric constant and good insulating properties. For such applications, the polymer may be dissolved in a solution which are in turn deposited on the substrate in droplet form to create a polymer thin film. Alternatively, a monomer may be dissolved in a solution and deposited by the droplet deposition process. The monomer thin film can then be polymerized following deposition. With suitable chemical precursors in a liquid form, the droplet deposition process can be used to fabricate a variety of thin films. Thin films of metal, semiconductor, insulator, polymer and ceramic can be created on a suitable substrate to form part of an integrated circuit device in microelectronics applications. The method and apparatus disclosed in this invention are suitable for all of these applications because of their ability to create uniform thin films of high quality at a sufficiently high speed for the mass production of these devices that are essential for the applications.
Although the droplet aerosol can be deposited directly onto the surface by the usual mechanisms of gravitational settling and Brownian diffusion, the deposition rate that is achievable is quite low and not adequate for the high speed production of integrated circuits. It has been discovered that the deposition rate is increased if the droplet mist is electrically charged.
In a previous U.S. patent application Liu et al., Ser. No. 08/706,664, now U.S. Pat. No. 5,996,640, issued Jun. 29, 1999 several methods to produce a charged droplet aerosol are disclosed. One method is to use an induction electrode placed in closed proximity to the atomizer nozzle. A high voltage is applied to this induction electrode to induce a charge on the surface of the liquid being atomized. The droplets formed this way are electrically charged.
In this invention, an alternative method is described which makes use of a high voltage electrode in a chamber having a compressed gas from a source upstream of the atomizer to produce a high concentration of ions (charged molecules) in the compressed gas. This ionized gas is then used to atomize the liquid to form droplets. Since ions in the compressed gas can collide with the droplets during atomization and impart their charge to the droplets, the resulting droplets are charged. This process has been found to be capable of providing a reproducible source of charged droplets for deposition on surfaces.
The present invention relates to a method and apparatus to atomize a liquid to form an aerosol of charged droplets or particles, means to concentrate an aerosol from a high volume stream into a small stream to form a concentrated stream of particles. The aerosol can then be deposited on surfaces for a variety of scientific and technical applications. The deposition process can be carried out at or near atmospheric pressure so that the need for a vacuum chamber and the associated vacuum equipment is eliminated. The method and apparatus provide considerable improvement over previous methods both in the speed of deposition, which is essential for the high volume production of large area flat-panel displays, and the lowered cost of the deposition equipment.
The aerosol generation and concentration apparatus has been devised specifically for producing thin films on large areas in order to manufacture large flat-panel displays and other integrated circuit devices on large flat surfaces using the aerosol deposition process. The droplet generating device with or without the concentrator can also be used as an electrostatic sprayer for spray coating applications. The electrostatic sprayer can also be used to produce a charged therapeutic aerosol to increase the deposition efficiency of such aerosols in targeted areas of the human lungs for medical applications. The method is applicable to both conductive and insulating liquids. This feature makes the method and apparatus described herein particularly useful in industrial spray painting applications where water-based conductive paint cannot be easily sprayed using conventional electrostatic spray guns.
Although the invention has been made with the specific purpose of fabricating thin films for flat-panel displays, the method and apparatus can also be used to create thin films for other purposes than flat-panels. One application where this invention is particularly suited is to deposit a ultraviolet (UV) absorbing coating on an underlying substrate on which images have been created by ink-jet and other forms of printing. The UV coating will allow such images to be preserved when used outdoors for advertising purposes. Other applications include optics coating to produce large area optical surfaces with special absorbing, emission, and/or transmitting characteristics.